|Non-Perturbative Particle Physics Phenomenology|
We are pursuing three major research areas. All of them are concerned with the problem of how particles behave (what their phenomenology is), once interactions can no longer be treated as a small effect (perturbation). A description of this research can also be found in our popular science blog. The latest news on our research or from the field can also be found on Twitter.
Foundations of gauge theories
So-called gauge theories represent the most common version of particle physics theories. Especially the standard model, but also gravity, and most of its speculated extensions belong to this category. While they have been extremely successful in the description of nature, several very fundamental issues of them are still poorly understood.
One of them is how to choose suitable coordinates to describe the elementary particles (called fixing the gauge). This problem, known as the Gribov-Singer ambiguity, arises because the quantum effects forbid to choose your coordinates purely locally, but you have to include knowledge of all space-time. Though this problem can be fixed in principle, we still lack practical solutions.
We attempt to resolve this problem by finding suitable averages over coordinate system ensembles such that the averages can be easier treated than any individual coordinate system. As a byproduct, such ensembles over coordinate systems create new symmetries, which we try to understand and exploit.
The QCD phase diagram
QCD is the theory of strong nuclear interactions, giving us stable atomic nuclei. It also governs the interior of neutron stars, the most compact stellar objects still dominated by particle physics and not only gravity.
Such ultra-dense systems are not easily accessible numerically. This is the so-called sign problem. As a consequence, no satisfactory description of the interior of neutron stars was yet possible using QCD. Thus, our knowledge of these stars is still rather speculative.
Non-simulation methods are a possible alternatives for the description of neutron stars. However, such methods usually involve various approximations. To check these, we perform simulations in slightly modified versions of QCD, where such simulations are possible. The aim is to find a suitable set of approximations which can be translated back to QCD.
Higgs physics in and beyond the standard model
The discovery of the Higgs boson has been one of the greatest scientific discoveries of the recent years. It is the last element of the standard model of particle physics, and a possible gateway to whatever lies beyond. With this discovery, the properties of the Higgs came into the focus of investigations.
The theory underlying the Higgs sector has plenty of enigmas so far. One is that it is not even clear, whether it a real theory, or whether new physics is mandatory to make it a real theory. The other is that there are subtle effects which are not yet accounted for, but which could provide distinct new signatures, like states containing several Higgs bosons.
To fully understand Higgs physics, we simulate generalized versions of the Higgs theory numerically. We determine the possible multi-particle states, and deduce from them predictions for experiments, like at the LHC. We furthermore use our understanding of gauge theories to infer how Higgs theories work. We also test various extensions of the standard model, whether they are possible candidates to complete the Higgs sector.